The present disclosure generally relates to configurations, components, assemblies and related methods of thermal barrier coatings, and in particular nanocomposite thermal barrier coatings with tough and soft or reactive phases in a strain tolerant microstructure.
Gas turbines are of prime importance in a range of industrial sectors, particularly for power generation and for propulsion of aircraft, marine craft, etc. The design of modern gas turbines is driven by the demand for higher turbine efficiency. It is widely recognized that turbine efficiency can be increased by operating the turbine at higher temperatures. In order to assure a satisfactory life span at these higher temperatures, thermal barrier coatings (hereinafter referred to as “TBCs”) are applied to airfoils and combustion components, for example, of the turbine using various techniques. Ceramic thermal barrier coatings within such turbines represent the predominant area of their development, playing increasingly key roles in providing protection against over-heating and oxidation of metallic components.
A key concern for turbines utilized in both power generation and propulsion applications is with harmful effects of ingested species or particulate, often referred to as “dust,” which can adhere to TBCs and damage them. Ingested dust melts during use of the turbine and typically forms a composition of molten dust referred to as “CMAS” (Calcia-Magnesia-Alumina-Silica). Calcia, magnesia, alumina and silica are the main ingredients of what is typically regarded as a family of particulate matter referred to as CMAS. However, other species of materials which may be referred to by the “CMAS” classification include volcanic ash, salt, and sulfur. Ingested CMAS induce degradation in both unprotected metallic alloys and ceramic coatings, such as TBCs. For example, a chief mode of failure of ceramic layers, and particularly TBCs, due to CMAS ingestion is loss of TBC compliance. Typically, the molten CMAS penetrates and fills the pores of the TBC structure. The penetrated CMAS then solidifies within the pores as the TBC cools. As another example, CMAS ingestion can damage ceramic layers, and particularly TBCs, by promoting sintering and thereby making them prone to spallation. Such degradation commonly arises when ingested particulate adheres to the coating and either creates a CMAS-rich outer layer or leads to diffusion into the coating of these oxides along internal grain boundaries, free surfaces (e.g., pores) or open defects. These oxides do not readily dissolve in the zirconia lattice, but tend to form vitreous phases, where they can accelerate sintering (particularly if significant levels of “liquid” phases are created).
One TBC category in industrial use is yttria-stabilized zirconia (YSZ) based TBCs, such as 7YSZ which offers chemical stability, low thermal conductivity and relatively high thermal expansivity that reduces coating-substrate thermal mismatch strains during heating and cooling. Air plasma spraying (APS) is widely used to produce such YSZ-based coatings. Unfortunately, conventional YSZ-based thermal barrier coatings deposited via APS have been shown to not include sufficient resistance to spallation when CMAS is ingested from the environment, as discussed above. Electron-beam physical vapor deposition (EBPVD) of YSZ-based TBCs has shown better spallation resistance against CMAS ingestion than APS applied coatings. However, although PVD coatings also provide attractive strain tolerance properties they tend to be relatively expensive and applicable to only relatively small components when compared with APS as the PVD processes requires a vacuum chamber and supporting equipment. Another newer technique to combat spallation resulting from CMAS ingestion involves TBC compositions with higher rare earth contents as compared to prior TBCs. These high rare earth TBCs are designed to react with ingested CMAS, and/or resist reactions with CMAS (or molten silicate), and thereby limit its penetration. These high rare earth TBCs, however, have much lower fracture toughness than conventional YSZ-based thermal barrier coatings, such as 7YSZ, and are thereby prone to thermo-mechanical stresses during engine operation.
As a result, a need exists for thermal barrier coatings and related methods that are resistant to CMAS ingestion (i.e., spallation resistant), include high strain tolerance, are scalable (i.e., compatible with large components), and are relatively inexpensive as compared with prior art thermal barrier coatings.